1. Field of the Invention
The present invention relates to an apparatus for determining position of a free piston such as reciprocating piston in hydraulic compressor, which has no mechanical connection with driving means like a crank shaft, and an apparatus for controlling reciprocating motion of the free piston.
2. Description of the Related Art
A free piston machine has a reciprocating piston with no mechanical connection with a driving crank-shaft. Rather, a linear motor or an alternator is often used to drive or load a free piston in its cylinder (such as the invention disclosed in reference 1). These machines are put into practical use as gas compressors, Stirling coolers, Stirling engines, and other free piston machines.
The free piston machine inherently lacks absolute piston displacement control of its free piston, because the free piston is not mechanically connected with a crank shaft. Therefore, its piston motion must be continuously monitored and controlled in order to obtain maximum stroke without noise and possible damages due to over-stroke. An ideal sensor in this application is non-contacting, has no moving parts, and has no need to have itself installed inside the case or pressure vessel of the free piston machine.
Hitherto, in prior arts, means are proposed, in each of which a permanent magnet is disposed on a free piston, and a Hall effect type sensor is disposed on a cylinder, and position of the free piston is determined by the Hall effect type sensor surveying the magnet moving with the free piston (reference 2 and reference 3). These methods are base on a principle, in which a magnetic sensor generates a wave type output signal when a permanent magnet passes across the magnetic sensor, and the wave type output signal has a peak when the permanent magnet approaches to the closest to the magnet sensor. Specifically, a free piston position signal is output when the wave type signal intensity exceeds a given allowable value.
Reference 1: U.S. Pat. No. 4,602,174
Reference 2: JPA 1989-31002
Reference 3: JPA2003-527591
However, these prior arts do not disclose concrete methods of how to detect stroke of a free piston and the average position of reciprocating motion of the free piston. It is principally possible to detect whether a free piston reaches a pre-determined stroke position, if a magnetic sensor is arranged so that strength of an output signal surpasses a set value when the free piston reaches a pre-determined stroke position, i.e. the maximum stroke position. However, in this method, in order to control each of variable strokes of the free piston, plural stroke set values and plural magnetic sensors are needed. Especially, it is virtually impossible to control continuously variable stroke of the free piston. Moreover, it is difficult to know how to detect the average position of reciprocating motion of the free piston with those prior arts.